GM Racing Developed NASCAR Impala SS in Virtual Wind Tunnel

DETROIT - The winds of change are blowing at gale force in the NASCAR Nextel Cup Series. The imminent debut of the Chevrolet Impala SS "Car of Tomorrow" and the impending introduction of GM Racing's all-new R07 small-block V-8 racing engine are signposts that point the way to a new direction in technology in America's most popular racing series.

One of the most powerful implements in GM Racing's toolbox is Computational Fluid Dynamics (CFD) - a mathematical simulation of the airflow around a vehicle. This emerging technology is influencing the design of both racing and production automobiles. CFD played a key role in GM Racing's analysis and development of a racing version of the Impala SS, the new-generation NASCAR race car that will make its competition debut in Bristol, Tenn., on March 25.

"The best way to describe CFD is a wind tunnel in a computer," explained Kevin Bayless, GM Racing oval track chassis/aero program manager. "CFD is a simulation that predicts the aerodynamic forces acting on a vehicle using computational methods rather than physical measurements. CFD allows engineers and racing teams to evaluate the effects of aerodynamic changes quickly on a computer screen rather than in a conventional wind tunnel."

First developed for aerospace and defense applications, CFD migrated to the civilian sector as supercomputers became available to run the complex programs.

"The CFD software requires a supercomputer because the number of calculations is immense," Bayless noted. "For example, our aerodynamic models typically have more than 10 million discrete data points that are used to calculate the forces. It really is rocket science."

The price of admission to this elevated level of technology is high, but the benefits of CFD are substantial. The sophisticated software makes it possible to "see" the invisible movement of air over a body.

"CFD allows us to visualize the flow to understand what the air is doing and where the aerodynamic forces are being generated," said Bayless. "That's simply not possible with a model or a full-size vehicle in a wind tunnel. CFD also allows us to test aerodynamics without a physical model, which can be helpful in the early stages of a design. We can analyze and compare various alternatives before actually building a prototype vehicle."

As the Car of Tomorrow becomes NASCAR's race car of today, the Impala SS will take center stage as Chevrolet's high-profile entry in Nextel Cup competition. CFD played a prominent role in reinforcing the identity of Chevy's new on-track representative.

"Although the Car of Tomorrow is highly regulated, it's not a spec race car," Bayless noted. "As a manufacturer, Chevrolet was able to define many of the details that separate the Impala SS from our competitors' cars. The headlights, grille, portions of the hood, and the tail were areas where GM Racing was able to incorporate Impala SS styling cues and brand identity. These areas are now fixed as part of the NASCAR Impala SS package."

The Impala SS race car's aerodynamics differ significantly from the Monte Carlo SS that has been the mainstay of Chevrolet's NASCAR program. GM engineers employed CFD to gain insights into the effects of the new body shape, front splitter, and adjustable rear wing on aerodynamic performance.

"NASCAR specifications require the Impala SS to be wider and taller than the Monte Carlo SS that it is replacing, so its frontal area is larger and its aerodynamic drag is approximately 10 percent greater," Bayless commented. "Initially the Impala SS will have about 15 percent less downforce than the Monte Carlo SS, which has been highly refined over the years. The change from a spoiler to an adjustable rear wing appears to reduce turbulence in the wake of the car, so when two cars are running nose-to-tail, the less turbulent air behind the first car should alleviate some of the aerodynamic push experienced by the trailing car."

Although CFD is a powerful engineering tool, it is not a replacement for conventional wind tunnels and track testing.

"The computing time required to run CFD programs is a limitation even with a supercomputer," Bayless noted. "A team can run through more tests in a day in a wind tunnel than can be done with CFD. At this point in its development, CFD supplements the testing that's done in wind tunnels and on race tracks. We share the information that GM Racing obtains through CFD with the Chevy teams. Given the level of competition in NASCAR today, it's vital that every team takes advantage of every available resource."